The diffusion profiles of small He bubbles along grain boundaries were studied by means of  in-situ  electron microscopy. It was found that the bubbles moved 2-dimensionally in the general grain boundary interface, with a misorientation around the [¯1¯22] axis. The square of the bubble migration distance along the boundary could be well fitted by means of random walk theory. The mean square of the migration distance was proportional to the time which was required to move that distance. It was concluded that these results provided clear quantitative evidence for the Brownian-type motion of the bubble along the grain boundary, and yielded the bubble diffusivity. For example, the diffusivity was equal to 5.6 x 10-18m2/s at 833K for a bubble diameter of 2.3nm. This diffusivity was much higher than that in the lattice matrix.

K.Ono, S.Furuno, S.Kanamitu, K.Hojou: Philosophical Magazine Letters, 1997, 75[2], 59-64